How to fix 'Couldn't modify partition map' error on Mac?

Data Recovery
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I’m trying to resize a partition on my Mac, but I keep getting an error message that says ‘Couldn’t modify partition map.’ This is really urgent as I need this space for my work. Has anyone faced this issue before? How did you fix it?

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Ugh, I’ve been there with this “Couldn’t modify partition map” error. Total pain, especially when you need that space for work stuff. Here’s what worked for me—breaking it down step by step:

  1. Backup Everything: First, make sure to backup all your data. This error can sometimes mess with your files, and you don’t want to lose anything important.

  2. Disk Utility: Open Disk Utility (Applications > Utilities > Disk Utility). Select the drive you’re trying to resize and run the “First Aid” tool. This checks and repairs any issues on the drive.

  3. Third-Party Software: If Disk Utility doesn’t cut it, you might need third-party software. I used Disk Drill, and it was a lifesaver. You can Download Disk Drill from their website. It has a feature for disk management that can sort out partition issues without wiping your data.

  4. Safe Mode: Sometimes, booting into Safe Mode can help. Restart your Mac and hold the Shift key until the Apple logo appears. This loads only necessary software, making it easier to modify the partition map.

  5. Terminal Commands: If you’re comfortable with the Terminal, it can be a powerful tool. Use the diskutil command to list and modify partitions. For example:

    diskutil list 
diskutil cs resizeStack <logicalvolumegroupUUID> <size>

    Replace <logicalvolumegroupUUID> and <size> with your specific values.

  6. Reformat: As a last resort, you might need to reformat the drive. This is why you backed up everything first. You can do this through Disk Utility by erasing the partition and setting it up again, but keep in mind this will wipe your data.

  7. Firmware Update: Ensure your Mac’s firmware is up to date. Sometimes these partition errors are due to outdated system software. Just go to the App Store or System Preferences and check for updates.

It’s always stressful when you’re dealing with disk issues, so take it step by step. Hopefully one of these methods will work for you. Good luck!

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Why even bother with partition resizing on a Mac? Apple’s Disk Utility is notoriously unreliable for anything advanced. You get errors like ‘Couldn’t modify partition map’ all the time. I’ve run into it before, and it’s maddening. Forget about it and use something like Disk Drill. It’s pretty good for these tasks, though some people whine that it’s pricey and can be slow. But at least it works. And you do get a fuller suite of tools, not just partition management.

Disk Drill Screenshot
Disk Drill Soft1536×912 80.7 KB

But hey, you could also look at Paragon Hard Disk Manager or GParted if you’re familiar with Linux. Just don’t expect miracles if your Mac’s drive is already a mess. Also, make sure your backup game is strong; these processes can screw up your data if something goes wrong. Don’t say I didn’t warn you!

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The error “Couldn’t modify partition map” usually crops up due to several reasons like file system corruption, incorrect partition scheme, or existing processes that are using the disk. Here’s a detailed guide on how to tackle this issue:

  1. Backup Your Data:
    Before you perform any disk operations, always backup your important data. Use Time Machine or any external drive to make sure your files are secure.

  2. Check Disk for Errors:
    Run Disk Utility to check and repair errors on your drive:

    • Open Disk Utility (you’ll find it in Applications > Utilities).
    • Select the disk you want to check from the left sidebar.
    • Click the First Aid button at the top of the window.
    • Let Disk Utility run its check and repair any detected issues.

  3. Unmount the Disk:
    Sometimes, partitions can’t be modified because they’re in use. Try unmounting the volumes:

    • In Disk Utility, select each partition on the disk and click “Unmount”.
    • Once unmounted, attempt the resizing operation again.

  4. Boot into Recovery Mode:
    If the above steps don’t work, you may need to use macOS Recovery to perform the operation:

    • Restart your Mac and hold down Command (⌘) + R until the Apple logo appears.
    • Once you’re in Recovery Mode, select Disk Utility from the utilities window.
    • Try repairing the disk again and perform the partition resize.

  5. Use Terminal for Advanced Commands:
    Sometimes, manual intervention via Terminal is required. Here’s how you can use diskutil to modify partitions:

    • Open Terminal (Applications > Utilities).
    • List all disks: diskutil list
    • Note the identifier of the disk you want to modify (e.g., disk0).
    • For detailed information: diskutil info disk0
    • Repair the disk and check the file system: diskutil repairVolume /dev/disk0s2 (replace /dev/disk0s2 with your specific disk identifier).
    • Now, attempt to resize again via Disk Utility or Terminal commands.

  6. Check for FileVault Encryption:
    If FileVault is enabled, it can sometimes cause issues with partition changes. Temporarily turn off FileVault:

    • Go to System Preferences > Security & Privacy.
    • Click the FileVault tab.
    • Click the lock icon to unlock it, then click Turn Off FileVault…

    Note that decrypting might take some time depending on the amount of data.

  7. Third-Party Tools:
    If you’ve tried the steps above without success, consider using third-party disk management software. One good option is Disk Drill, which has a robust set of tools for diagnosing and fixing disk issues. You can download it at Disk Drill.

  8. Free Up Disk Space:
    Low disk space could be a bottleneck. Ensure there’s enough free space on the disk:

    • Remove unnecessary files and applications.
    • Empty the Trash.
    • Ensure at least 15-20% free space for optimal functionality.

  9. Run in Single-User Mode:
    Any persistent issues might require running commands in single-user mode:

    • Restart your Mac and hold down Command (⌘) + S during reboot.
    • Once in single-user mode, type fsck -fy and press Enter.
    • Repeat the command until no errors are reported.
    • Type reboot to restart normally.

  10. Erase and Reformat the Disk:
    If nothing works, the last resort is to erase and reformat the disk. This will remove all data:

  • In Disk Utility, select the problematic disk.
  • Click Erase and choose the appropriate format (APFS for newer Macs, Mac OS Extended (Journaled) for older ones).

After erasing, you can then reinstall macOS and restore your data from the backup.

Remember, using Disk Drill or other professional tools might simplify the process if you’re an advanced user but always make sure to have backups regardless of the method chosen.